Fix LoD and Add logging

[zhonghan721]

• Remove use of argMultimap.getPreamble().isEmpty() from
  CustomerAddCommandParser, UserUpdateCommandParser,
  UserLoginCommandParser
• Add logging to CustomerFindCommandParser

Closes #527

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (534974f) 83.90% compared to head (6552778) 83.91%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #529      +/-   ##
============================================
+ Coverage     83.90%   83.91%   +0.01%     
- Complexity     1010     1011       +1     
============================================
  Files           121      121              
  Lines          3025     3027       +2     
  Branches        360      360              
============================================
+ Hits           2538     2540       +2     
  Misses          389      389              
  Partials         98       98              

Files	Coverage Δ
...ogic/parser/customer/CustomerAddCommandParser.java	100.00% <100.00%> (ø)
...gic/parser/customer/CustomerFindCommandParser.java	100.00% <100.00%> (ø)
...ress/logic/parser/user/UserLoginCommandParser.java	100.00% <100.00%> (ø)
...ess/logic/parser/user/UserUpdateCommandParser.java	95.45% <100.00%> (ø)

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[juliusgambe]

LGTM